In known optical transmission systems, an optical element made of a material containing silicon transmits a multiplexed optical signal composed of optical signals of a plurality of different wavelengths (Refer to Japanese Laid-Open Patent Publication Nos. 2013-041143, 2009-139734, 2013-157722, and Japanese National Publication of International Patent Application No. 2013-513825, for example). Such optical transmission systems can be implemented, for example, using the silicon photonics technique. According to Wikipedia published on the Internet, “Silicon photonics is the study and application of photonic systems which use silicon as an optical medium”.
For example, the optical transmission system transmits an optical signal between electric circuits such as CPUs and LSIs. The term CPU is an abbreviation of central processing unit. The term LSI is an abbreviation of large scale integration. The optical transmission system is also referred to as an optical interconnect, for example.
The optical signal transmitted using an optical waveguide is less subjected to degradation in waveform than the electric signal transmitted using a metal wire. Therefore, the transmission of the optical signal allows for a larger transmission line capacity than the transmission of the electric signal.
In the above-mentioned optical transmission system, a light source may be formed of a compound semiconductor such as gallium arsenide, for example. Further, the light source may be disposed near another optical element. Therefore, the light source tends to become hot, degrading itself due to thermal stress and the like. This lowers the optical output level of the light source. For example, as the optical output level of the light source that outputs light of certain wavelength is smaller, the transmission rate, that is, the amount of information transmitted in the light of the certain wavelength per unit time decreases. Accordingly, the transmission rate that is the amount of information transmitted in the multiplexed optical signal per unit time also decreases.